rev-lib: Make it easy to do rename tracking (take 2)

rev-lib: Make it easy to do rename tracking (take 2)

prune_fn in the rev_info structure is called in place of
try_to_simplify_commit. This makes it possible to do rename tracking
with a custom try_to_simplify_commit-like function.

This commit also introduces init_revisions which initialises the rev_info
structure with default values.

Signed-off-by: Fredrik Kuivinen <freku045@student.liu.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>